Wireless communication systems in which wireless terminals (e.g. fixed or mobile) communicate with fixed base stations via a wireless channel are well known in the art. Since the wireless spectrum bandwidth is limited, various techniques have been developed to allow multiple wireless terminals to share the limited bandwidth. One technique is time division multiple access (TDMA) in which wireless terminals share a single frequency but are separated in the time domain with each wireless terminal being assigned a time slot. Another technique is frequency division multiple access (FDMA) in which wireless terminals are separated in the frequency domain, with each wireless terminal being assigned a particular frequency.
The present invention is concerned with another technique for sharing limited spectrum, namely code division multiple access (CDMA). In a CDMA system, the signals of all wireless terminals share a frequency band and are sent together in time. The signals of different wireless terminals are differentiated using codes. Each wireless terminal is assigned a unique code (S) (also known as a spreading code or signature). The signal (d) transmitted from a particular wireless terminal to the base station is combined with the wireless terminal's unique code S to generate an encoded signal over a wide band of frequencies. If there are K wireless terminals (1, 2, . . . K), the signal of the kth wireless terminal is represented as {dk(•)} and the code of the kth wireless terminal is represented as Sk. The transmitted signal for the kth wireless terminal at the ith time instant is represented as dk(i)Sk. Thus, the combined signal for all the wireless terminals received at the base station is d1(i)S1+ . . . +dK(i)SK. The codes are ideally designed to be orthogonal such that the signal of the kth wireless terminal may be extracted at the base station by multiplying the received signal with the wireless terminal's unique code Sk. Signals originating at the base station are also combined with a code prior to being transmitted to each wireless terminal. Upon receipt of the signals, the wireless terminal extracts the signals from the base station by multiplying its received signal by appropriate codes. CDMA technology is well known in the art of wireless communications and will not be described in further detail herein.
One problem with CDMA systems is inter-user interference (also known as multiple access interference (MAI)) which results from the leakage of signals of one wireless terminal into the signal of another wireless terminal. This leakage is due to several factors. First, there is multipath propagation which results in the same signal being propagated along multiple paths with different time delays. Moreover, the codes are usually not perfectly orghogonal in order to be able to accommodate a large number of terminals. This non-perfect orthogonality itself also contributes to inter-user interference. Also, received signals may not all be perfectly synchronized in time, which also contributes to the inter-user interference.
In order to deal with the interference problem, current systems generally perform complex operations at the receiver in an attempt to remove this unwanted interference. One approach of dealing with interference is single user processing, in which a receiver uses only knowledge of a single signal (e.g. the signal associated with a single wireless terminal) to process the incoming signal. A more sophisticated approach is multi-user processing, wherein the receiver uses knowledge of all signals in order to demodulate them all in a joint fashion. This multi-user approach is generally only performed at base stations because of the significant processing overhead which it requires.